fix: finalize libuv

[eric-wieser]

This PR ensures that libuv objects are cleaned up by finalize_runtime_module().

Closes #7753

[leanprover-community-bot]

Mathlib CI status (docs):

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• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase 9afe5ccae3a4b7d5e49dc73a4089de64570282b9 --onto 0f4459b42c87d7712391e6d87160da3077fff671. You can force Mathlib CI using the force-mathlib-ci label. (2025-06-03 22:41:02)

[algebraic-dev]

I think that this PR will probably close #7753 right? Would you mind if I make some modifications to your PR to address the remaining problems?

[eric-wieser]

@algebraic-dev, I am very happy for you to take over here!

[algebraic-dev]

I was trying to run the initializers and finalizers in an infinite loop, and sometimes it gives me a segfault. I changed the lthread call to a pthread call just to check if it was related, and it seems like it is. when I'm using raw pthreads, it just doesn't segfault.